Snappy, spirited and shot through with the pangs and pleasures of leaving childhood behind, Lady Bird is a sharp-witted solo first feature by actress and now writer-director Greta Gerwig. The film abounds with pinpoint insights into its mildly rebellious heroine's hunger to shed the restraints of home and Catholic school and bust into an independent life, and does so with a wealth of keenly observed detail. Modestly scaled but creatively ambitious, it succeeds on its own terms as a piquant audience pleaser.

Gerwig opens her debut with a Joan Didion quote about the director's hometown, where the action will be set: “Anybody who talks about California hedonism has never spent a Christmas in Sacramento.” Indeed, the only time in recent memory that the California state capital has appeared in a feature film was when Gerwig's character in Noah Baumbach's Frances Ha briefly returned home from New York City.

Set in 2002-2003, the central character's final year in high school, the film makes plenty of little jokes about the stultifying limitations of the mostly middle-to-upper-middle-class neighborhoods in which the action is set, not to mention the rules imposed by the Catholic school Lady Bird (Saoirse Ronan) attends. To be sure, Lady Bird is not her real name, but simply one of the many methods Christine McPherson employs to distance herself from, and piss off, her parents, Marion (Laurie Metcalf) and Larry (Tracy Letts).

Any teenager with a mother like Lady Bird would squirm to get away, too. Hypercritical, demeaning and determined that her daughter not exceed her own modest status in life, Marion is a constant pain to be around, getting in Christine's face about matters big and small (Mom refuses to acknowledge her made-up name). But Gerwig paints this sad woman in an understanding and dimensional way, as someone forced to work double shifts at the hospital to make up for her husband losing his job and his inability to find a new one. She's a limited woman, to be sure, and one keenly aware of the emotional desolation that lies ahead once her daughter is gone.

The Catholic school experience is rendered with hilarity but, again, without meanness and even with a sprinkling of generosity. The senior nun on the staff (the wonderful Lois Smith) cuts through the BS to be real with Lady Bird, and great fun is had with the school's theater productions; when the original faculty director of The Tempest bails out, he's replaced by the school's football coach, a loudmouth who rambunctiously illustrates how the Shakespearean drama will be staged by diagramming a chalkboard with X's and O's and firmly drawn arrows.

And what is senior year in high school without some sexual adventuring? Lady Bird initially sets her sights on handsome thespian Danny O'Neill (Lucas Hedges), but when that goes off the rails she cozies up to too-cool-for-school iconoclast Kyle (Timothee Chalamet, so good in the upcoming Call Me by Your Name). Gerwig and her actors skillfully limn the hesitations and thrills of first wading, then diving into sexual waters, catching along the way lovely fleeting moments of awkwardness, pretenses, uncertainties and what-the-hell abandon that can accompany early desire, lust and eagerness to get on with life.

More than anything, though, Lady Bird wants to get out of Sacramento. Even as her mother insists she go to college nearby, Lady Bird secretly applies to a school in New York (she's not a great student but, immediately post-9/11, it's supposedly easier to get in somewhere there). Money is another issue, but that's a bridge to be crossed when necessary.

The film flies right along, passing so quickly from scene to scene that it occasionally risks becoming akin to sketch comedy. Sometimes you can almost sense Gerwig sorting through a pile of notes looking for a good gag or line that will get her onto whatever comes next, then diving in again for a snappy follow-up. But the sense of genuineness and authenticity trumps any lasting feel of glibness, so that the dozens of short scenes gradually accumulate to provide a satisfyingly full sense of Lady Bird's heart and mind. Even when the film is glib, it is mostly also true.

So while Lady Bird is neither really a rebel (she has only modestly dyed hair) nor a proponent of an explicit cause, she is determined to assert herself sufficiently to continue her search for what she wants in life and not capitulate to the limitations her worn-down mother tries to impose. This makes her mild-mannered, now professionally sidelined father a more interesting character than he looks to be at first. Whereas Marion, for all her faults, is an obvious take-charge type, Larry has lost any say over any matter due to his protracted joblessness. But Lady Bird clearly loves her dad, and he ends up having rather more to do with how his daughter's life proceeds than does her hard-charging mom. Both Metcalf and Letts are superb in their roles.

Although Lady Bird is less original, there's still a sense here of the urgency of youth, of being as-yet unformed and of impulsive impudence that call to mind Francois Truffaut's 1959 classic The 400 Blows. It certainly seemed that one was learning a great deal about Gerwig herself while watching her presumably self-revelatory performances in her films with Noah Baumbach or, before that, in her collaborations with Joe Swanberg (the two shared writing and directing credit on Night and Weekends in 2008).

But one takes away a good deal more from this film thanks to the marvelous performance by Ronan, who just seems to keep getting better all the time. She handles her many throwaway lines with great aplomb — they always land — and yet the undercurrent of uncertainty about the future (her schooling, money, parents, would-be boyfriends, career) is always near. No matter her faux pas, fibs, missteps and deceptions, you're with her all the way, and her sharpness of mind is such that it promises a bright future. Gerwig has provided the truth of that prediction with her work here.